In order to increase the efficiency and the performance of gas turbine engines so as to provide increased thrust-to-weight ratios, lower emissions and improved specific fuel consumption, engine turbines are tasked to operate at higher temperatures. The higher temperatures reach and surpass the limits of the material of the components in the hot section of the engine and in particular the turbine section of the engine. Since existing materials cannot withstand the higher operating temperatures, new materials for use in high temperature environments need to be developed.
Ceramic matrix composites have been developed as substitutes for the high temperature alloys. The ceramic matrix composites (CMCs) in many cases provide an improved temperature and density advantage over metals, making them the material of choice when higher operating temperatures and/or reduced weight are desired. CMCs have relatively low thermal conductivities and are thus well suited for use in high temperature environments for long periods of time.
Silicon carbide and silicon nitride ceramics undergo oxidation in dry, high temperature environments. This oxidation produces a passive, silicon oxide scale on the surface of the material. In moist, high temperature environments containing water vapor, such as a turbine engine, both oxidation and recession occurs due to the formation of a passive silicon oxide scale and subsequent conversion of the silicon oxide to gaseous silicon hydroxide. To prevent recession in moist, high temperature environments, environmental barrier coatings (EBC's) are deposited onto silicon carbide and silicon nitride materials. As such, CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment.
Additionally, CMC components in the hot gas are film cooled, particularly in components for use within the hot gas path. For example, film holes may be formed in the CMC component via laser drilling. However, this laser drilling results in deposition of silicon liquid metal “splatter” on the surface of the EBC, which upon freezing, undergoes volume expansion of silicon and subsequently damages the EBC coating.
As such, a need exists for an improved method of forming holes (e.g., film cooling holes) in a coated CMC component.